Hybrid vehicles have recently been of great interest as environment-friendly vehicles. The hybrid vehicles are now partially commercialized.
A hybrid vehicle has, as its motive power sources, a DC (direct current) power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. More specifically, the engine is driven to secure the motive power source and a DC voltage from the DC power supply is converted by the inverter into an AC voltage to be used for rotating the motor and thereby securing the motive power source as well.
Japanese Patent Laying-Open No. 2001-157304 discloses a motor drive system for a hybrid vehicle. The motor drive system estimates demagnetization of a permanent magnet of an electric rotating machine from the temperature of the permanent magnet according to data used for controlling the hybrid vehicle.
The conventional method, however, estimates demagnetization from the temperature of the permanent magnet which is estimated according to the control data for the hybrid vehicle, resulting in a problem that the demagnetization cannot accurately be estimated.